Consumer demand for Tesla’s Powerwall home batteries has been “staggering” according to CEO Elon Musk, though he expects utilities and businesses to become the bigger customers over time.

Tesla announced the Powerwall last week, letting customers sign up to reserve one on the company’s website. The battery comes in two flavors: A 7 kWh model costs $3,000 and is meant for daily cycles—for instance, by solar panel owners trying to get off the grid—while a 10 kWh model costs $3,500 and serves as a backup solution, optimized for weekly cycling. (Those prices don’t include inverter or installation costs.)

In an earnings call, Musk said customers have reserved 38,000 Powerwall batteries in less than one week, Ars Technica reports. Granted, those customers don’t have to put down any money right now, and are merely holding a place in line for the launch this summer.

Musk acknowledged that living off the grid with a Powerwall will likely be less cost-effective than staying on the grid and selling excess energy back to the utility company. “Some people want to go off-grid on principle,” Musk said, but doing so will cost more in the U.S., where electricity is fairly cheap. Powerwall should make more sense in countries like Germany and Australia, where solar use is more widespread and energy is pricier, he said. Ars also points out that with average home energy use, the backup battery may only last a few hours.

In the future, Tesla expects five to ten times more sales (in terms of megawatt hours) from businesses and utility companies. The former can store energy on a much larger scale to avoid peak demand charges, and the latter can help utilities better-manage their grid and store excess renewable energy.

Why this matters: Tesla’s vehicles aren’t necessarily more cost-effective than a gas-powered car, either. So even if the Powerwall isn’t more cost-efficient than staying on the grid or more useful than a gas-powered generator, the new batteries serve as a sort of statement about reducing fossil fuel dependence. It’s no surprise that Tesla has already found thousands of people who want to get on board.

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